(1) Field of the Invention
The invention relates to an apparatus and a method for fabricating hologram, and more particularly to an apparatus and a method that fabricate hologram by controlling exposing position of the hologram through a micro-display device.
(2) Description of the Prior Art
Digital holography technology is a technique based on computer technologies, laser holographic techniques, digital image processing techniques, precision optical control techniques and laser optical fabrication techniques. Through the digital holography technology, a novel picture may be produced that is radically different from the conventional printing pictures. It can generate a new type of picture with dynamic spatial variations. These new type of pictures have a great impact to packaging materials and printing industry. It has a great industrial application potential and commercial value.
Referring to FIG. 1 for a typical digital hologram lithographic system 100. It includes a coherent light source 120, a light splitting unit 140, a telecentric system 160 and a photoresist sheet holding station 180. The coherent light source 120 (such as a laser light) generates a coherent light CO which is refracted and reflected by the light splitting unit 140 to become a first coherent light C1 and a second coherent light C2 that are parallel with each other. The two coherent lights C1 and C2 project to the telecentric system 160, and are converged on the photoresist sheet holding station 180. When a photoresist sheet 200 is mounted onto the photoresist sheet holding station 180, due to the two coherent lights C1 and C2 have the same frequency, the interference stripes being generated do not change with time. Hence a steady exposed pattern may be provided by transferring the interference stripes accurately to the photoresist sheet 200.
As shown in FIG. 1, the light splitting unit 140 includes a light splitter 142 and a reflector 144. After the coherent light C0 projects to the light splitter 142, a portion of the light is reflected by the light splitter 142 and the rest of the light passes through the light splitter 142 to become the first coherent light C1 and the second coherent light C2 of different travel directions. The first coherent light C1 directly projects to the telecentric system 160, while the second coherent light C2 is reflected by the reflector 144 and projects to the telecentric system 160.
In the single lithographic process, the digital hologram lithographic system 100 processes exposure for merely one pixel of the photoresist sheet 200. To fabricate the entire hologram, the photoresist sheet 200 on the photoresist sheet holding station 180 must be moved to position every pixel of the photoresist sheet 200 individually for exposing.
Hence the time to finish the hologram lithographic process includes the exposing time of the photoresist sheet 200 and the time required to move the photoresist sheet holding station 180 to position the photoresist sheet 200. The time for exposing the photoresist sheet 200 is determined by the intensity of the coherent light source 120 and light absorption amount of the light splitting unit 140 and the telecentric system 160. Namely, the greater the energy of the coherent light source 120, the lower the light absorption amount of the light splitting unit 140 and the telecentric system 160 becomes, the light energy receiving amount on each unit area of the photoresist sheet 200 is greater, and the exposing time needed is shorter.
On the other hand, the time required to move the photoresist sheet holding station 180 to position the photoresist sheet 200 is determined by each time the photoresist sheet holding station 180 is moved to position the photoresist sheet 200 and the moving times. In general, when the photoresist sheet holding station 180 is moved to position the photoresist sheet 200 to a next spot, it is not stabilized instantly. Instead, it has to experience a short period of damping before stabilizing to process the next exposing. Moreover, a hologram of a VGA specification usually has 640×480 pixels. Hence the photoresist sheet holding station 180 has to be moved more than 300,000 times to finish the fabrication of the hologram. As a result, a great portion of the fabrication time is spent on positioning the photoresist sheet 200.
Therefore, how to shorten the time of processing hologram lithography and reduce the processing cost and improve fabrication speed are important issues in the application of the holographic technology.